I have a flashcart, and my brother does not, so naturally, I try to download games with good download play for both of us. I've only come across a few games that are actually fun in download play multiplayer. So far, here's my list:
Mario Party DS
Tetris DS
Meteos

(also, my brother owns MPH, MKDS, Jump! Super Stars, which was actually a gift from me, and some other random DS games)

Pac Man VS. is absolutely awesome, and available as a Download Play game. Besides that, Namco Museum has even more excellent Download Play content. You can send full versions of any of the collection's games to the other DS. Everybody loves Xevious, right? Xevious for everybody!